1. Field of the Invention
This invention relates generally to the forestry industry, and specifically to the field of woodworking machines. In particular, the invention relates to woodworking knives used in these machines, and to clamping assemblies that secure the knives.
2. Description of the Related Art
One of the main tasks in forestry industries is the processing of trees into finished wood products. Tree processing begins with harvesting live trees and cutting the trees into logs. Logs then are processed in various ways, according to the desired end product. Thus, forestry industry operations can be significantly affected by control over, and improvement upon, log processing steps and the machines used in these steps.
In the pulp or oriented strand board industries, logs are processed by machines designed to turn solid logs into chips or wafers. Examples of these machines include chippers (in both disc and drum forms), waferizers, and stranders. To perform their respective functions, these machines typically employ one or more knives mounted to a moving base such as, for example, a rotating disc or drum. Wood is processed by moving it into the path of the rotating knives, the blades of which contact the wood at a particular depth and orientation. This contact results in the formation of chips, shavings, wafers, or strands.
In the sawmill industry, logs are processed by machines designed to chip away certain portions of the logs to form rough lumber, with wood chips as a byproduct. Examples of these machines include chipper canters, chipper edgers, and chipper slabbers. Rough lumber can be further processed by planers to yield finished lumber, with wood shavings as a byproduct. All of these machines also employ knives, which are positioned to result in the formation of a cut or planed surface on the lumber.
In the veneer industry, logs are turned on veneer lathes to yield veneer sheets. These sheets can be used for manufacturing plywood or laminated veneer lumber. Veneer lathes also employ knives. However, in a lathe, wood is removed from a log not by repetitive contact of moving knives, but rather by bringing a rotating log into contact with a stationary knife mounted to the lathe.
Regardless of its use in a particular application, a woodworking knife can in time exhibit wear, resulting in dullness and even structural failure. Also, a worn knife may not cut wood effectively, resulting in wood chips or veneers having inconsistent size or shape. Thus, wood processing operations, such as a sawmills, seek to properly maintain their woodworking knives and, in particular, sharp knife edges.
Various methods for maintaining a sharp knife edge have been developed. In one method, a knife is removed from the machine, sharpened by grinding, and remounted to the machine. A disadvantage of this method is that, after grinding, the knife may have a different (and unwanted) size or shape. Thus, a knife sharpened this way usually requires careful positioning and aligning upon remounting to the woodcutting machine; knife alignment may be particularly important in machines having configurations of multiple knives. An incorrectly positioned knife tends to negatively affect the cutting properties of the machine, diminishing or canceling the effects of sharpening the knife. Also, grinding a knife edge to a precise and accurate shape can require costly, time-consuming techniques.
Another method for maintaining knife edges is the use of reversible knives. A reversible knife is manufactured with multiple edges (often two) and designed such that when one edge becomes worn from use, the knife can be removed from its mounting assembly, rotated or flipped about a symmetrical plane, and remounted, thereby exposing a fresh edge. Reversible knives are generally disposable: when all edges of a knife are worn, it is replaced with a new knife. This method overcomes the disadvantages inherent to grinding discussed above.
However, there are disadvantages to using reversible knives. Precise positioning of a reversible knife, whether upon initial mounting or reversal, can be difficult to achieve. Mounting is often done by hand; thus, the mounted position may be influenced by human error. Mounting errors can be exacerbated when a machine's knife mounts are not readily accessible, such as when mounts are located behind other components of the machine or when mounts are at a height difficult to reach. Moreover, in some applications reversible knives experience highly asymmetric and torsional loads. These loads can overcome the clamping forces holding knives to their mounts or cause knives to displace or fracture. U.S. Pat. Nos. 6,058,989 and 7,159,626 address some of these disadvantages.
U.S. Pat. No. 7,140,408 describes a reversible knife in which the chip guiding surface includes a reentrant portion. A feature of the knife that indexes a mounting assembly is located on this reentrant portion. This design is said to alleviate wear of the indexing feature when exposed to chip cutting. A disadvantage of this design occurs during chip forming and guiding. Chips cut by the knife edge are then guided along a chip guiding surface. However, when the chips reach the reentrant portion, they may lose contact with the knife until they have passed over the indexing feature. Once the indexing feature is passed, the knife has a deflecting ridge disposed at a large angle relative to the motion of the chips. As a result, the chips can fracture or splinter when hitting the deflector ridge. Also, the indexing feature of this knife is located at a significant distance from the cutting edge. Thus, if the knife is subject to a high amount of loads during cutting, the positioning of the indexing feature at a significant distance from the cutting edge may lead to breakage. That is, the distance of the indexing feature from the cutting edge creates a moment arm for forces applied to the cutting edge, which increases the stresses at the indexing feature. Further, the concave form of the indexing feature reduces the amount of structural material at this area of high stress, making it more prone to breakage. The thickness of the knife must be increased to compensate for this weakness. Although increasing the thickness of the knife can diminish torque breakage, this increases the material cost of the knife.
Another disadvantage of reversible knives is they tend to be manufactured from higher-quality materials and under stricter manufacturing tolerances than other woodcutting knives. While this yields durable knives with long-lasting sharp edges, it increases the costs of manufacturing the knives. In particular, the material cost can be significant. Thus, even a slight reduction in the amount of material in a reversible knife can result in a greatly reduced cost per knife. This can reduce the cost of operating woodcutting machines, particularly those configured to use multiple knives.
One way to reduce the amount of material in a reversible knife is to use a low-volume design. Such a design should have a compact form with small subsidiary surfaces, which are surfaces that are not directly utilized to cut, form, or guide chips. An example of a subsidiary surface is one that is used solely for clamping a knife to a mounting assembly. An example of a large (and disadvantageous) subsidiary surface is the indexing feature of the knife described in U.S. Pat. No. 7,140,408.